are two 4x4 equal to 4x8

huangyhg

are two 4x4 equal to 4x8
if two 4x4 posts are joined by bolt/4ft and strong adhesive will the resulting 4x8 compare with a 4x8 one piece?
  
almost, as the area, section modulus and moment of inertia of the two 4x4's are all slightly less than the 4x8.
the two 4x4's give a section of 3.5 x 7, where the 4x8 is 3.5 x 7.25, slightly larger.
mike mccann
mmc engineering
one other thing i would say about this is that you might want to include some kind of factor to account for slippage of the bolts and/or creep of the adhesive.  both of these will reduce the composite action from 100%
you can use an adhesive to improve composite action.  reduced section properties as per m^2-48 are likely most significant, if an issue, the weak axis kl/r hasn't changed.
dik
you might consider bolting at 2'-0'' oc.  the glue may or may not help - depends on the type.
i often use something like 75% of full value??!!
well, the nds does require a factor of 0.6 in the column stability equation of built-up wood   
yes - structuraleit is on the right track.  the other comments above reflect, perhaps, the "real" behavior of a bolted and glued pair of 4x4's is almost identical to the 4x8.
however, the nds, in section 15.3, has a kf factor that applies to the column stability factor cp which reduces the column capacity (in some cases 0.6 per structuraleit above).
if your column is free-standing then this factor applies.
if you column is within a wall, where sheathing is present to eliminate any bending that would cause horizontal slip shear in the plane between the two 4x4's, then the kf factor can be ignored.

we are all assuming aayjaber is wanting it as a 4x8 post.
what if he/she wants it as a 4x8 beam?
if that is the case, you could perhaps do it, but i'd be worried about the glue/bolt scenario being able to withstand the 3v/2a for an extended (if any) period.  glulam beams can do it, but that is at a controlled manufacturing facility and the materials and glue have  been tested.
chipb,  your comment is right i want to use the two posts glued and bolted as a ridge for a roof with 12 ft span, connecting the tops of two gable walls.  it is a one story so there are no other loads on the beam except the dead load. yes the beam will have to play well with the rest of the structure against wind and seismic activity.
i'm assuming the client is wanting to go with a post and beam look.  unless your order your beam from a glulam mfr, i'd stay away from this 2-4x4 scenario.
try to talk them into either an lvl or you could do a flitch plate beam out of rough sawn lumber.  
i've had this in my library for a while: